1. Field of the Invention
The present invention relates to an engine control device for detecting misfire of an engine and, on detection of misfire, performing counter-misfire control by, for example, issuing a warning or correcting the fuel injection quantity.
2. Description of the Related Art
Incomplete combustion of fuel in engine cylinders is an immediate cause of inconveniences such as deterioration in exhaust gas characteristics, and therefore, the regulations regarding OBD (On-Board Diagnosis) require that a motor vehicle be provided with the function of notifying the driver of the occurrence of misfire, which is a cause of incomplete combustion. The misfire detection is also utilized for various control purposes to avoid the combustion being deteriorated. In the circumstances, there has been a demand for a misfire detection device capable of accurate detection of misfire.
As conventional misfire detection devices, there has been known a device using a misfire detection method based on the period of crank angle (CA), for example. In this method, on the basis of the output from a crank angle sensor, an average angular acceleration is calculated with respect to every combustion cycle corresponding to 180° CA, and the calculated average angular acceleration is compared with a predetermined misfire criterion value to determine whether or not misfire has occurred. The period of the combustion cycle corresponding to 180° CA is, however, affected by external factors other than deterioration of combustion due to misfire, such as reactive movement of the engine during travel on a rough road or the input of load on the engine by an air conditioner etc., and such factors cause error of the average angular acceleration as a result. Accordingly, as measures against the error, the misfire detection is carried out based on three parameters, namely, the absolute value of average angular acceleration, and deviations of the absolute value from the average angular accelerations of preceding and succeeding combustion cycles.
Despite the measures, the conventional misfire detection device is still unable to provide satisfactory accuracy in misfire detection. For example, at the cold start of the engine, catalyst W/U (warming up) control is performed in which activation of the catalyst is accelerated by retarding the ignition timing and making the air-fuel mixture lean. In such an operating state, the engine speed varies greatly due to poor combustion, and also the average angular acceleration of each cycle is strongly influenced by the immediately preceding cycle. With the above misfire detection technique, therefore, it is difficult to discriminate between normal combustion and misfire, and thus accurate misfire detection cannot be expected.
Also, where the catalyst W/U control is performed on the engine using a heavy fuel, which is a fuel with very low volatility intended for use in high-temperature districts, the air-fuel ratio shifts toward the lean side as the amount of fuel adhering to the intake ports increases, deteriorating the combustion. To cope with such heavy fuel, therefore, during the catalyst W/U control, combustion control is carried out in which the fuel injection quantity and the ignition timing are corrected by using the average angular acceleration as an index of combustion, to thereby stabilize the combustion. However, if the calculated average angular acceleration itself contains error as mentioned above, then it is not possible to carry out appropriate correction matching the combustion deterioration level, causing inconveniences such as delay in activation of the catalyst or deterioration in exhaust gas characteristics.
In view of the foregoing, there has been proposed a misfire detection device which is adapted to detect misfire based on an amount of rotation variation during the combustion cycle of each cylinder, as disclosed in Unexamined Japanese Patent Publication No. 2000-205035, for example. In this misfire detection device, the amount of rotation variation during the combustion cycle of each cylinder is calculated based on the time period required for every 30° CA, and if the deviation of a currently calculated rotation variation amount from the rotation variation amount calculated three ignitions (360° CA) before is greater than a misfire criterion value, it is judged that misfire has occurred.
However, in the misfire detection device disclosed in FIGS. 3 and 4 of Unexamined Japanese Patent Publication No. 2000-205035, misfire of a current combustion cycle is determined with the use of the rotation variation amount of the combustion cycle which is three ignitions before the current cycle. Accordingly, the determination result not only reflects the current combustion but is influenced by the combustion three ignitions before.
Specifically, when the current combustion is slightly poor but not to such an extent that misfire is caused, it may possibly be judged that misfire has occurred, if the combustion three ignitions before was very satisfactory and thus the deviation between the rotation variation amounts is greater than the misfire criterion value. Conversely, even when the current combustion is so poor that misfire is caused, it may possibly be judged that no misfire has occurred, if the combustion three ignitions before was poor but not to such an extent that misfire is caused and thus the deviation between the rotation variation amounts is smaller than the misfire criterion value. Consequently, in such situations, the OBD fails to notify the driver of the occurrence of misfire. Also, where the combustion control, as measures to cope with heavy fuel, is performed during the catalyst W/U control, a problem arises in that the fuel injection quantity and the ignition timing fail to be appropriately corrected.